This invention relates primarily to furnaces of the type which burn conventional hydrocarbon fuels, such as gas, or fuel oil. Such furnaces conventionally include an air casing which provides a substantially vertical path for the room air to which heat is to be exchanged, an air blower, a combustion chamber for the hydrocarbon fuel and, associated therewith, a primary heat exchanger.
The combustion gases of such fuels, usually all vented to the flue, include roughly 10% water vapor. The sensible heat of the combustion gases so vented may be somewhere in the region of 450.degree. F. However, the water vapor includes latent heat, that is, the heat which may be realized when the water vapor is condensed.
None of the latent heat is extracted in using conventional furnaces, although its theoretical recoverability is well known.